The present invention relates to a diode tuned resonant circuit comprising at least two variable reactance diodes.
Tuned resonant circuits are frequently used in oscillators to vary the frequency of the oscillator in response to an applied voltage. Variable frequency oscillators are used in a variety of applications. One such application is in a frequency agile modem which connects a computer to a multi-channel network. Each channel is assigned a distinct frequency band within which data may be transmitted and/or received.
For modulation and demodulation operations, a modem typically uses a local oscillator to generate a signal of known frequency. The frequency of this signal must be very stable to accurately and reproducibly code and decode the communication data. Since a frequency agile modem may communicate over several frequency channels, it is desirable for that modem to have a local oscillator which can generate any one of several frequencies. In addition, if the modem is of the type which transmits and receives on different frequency channels, it is often desirable for the modem to contain two local oscillators, one for transmission, and one for reception operations.
One means for providing stable multi-frequency signals is to use an oscillator circuit which contains a crystal for each respective communication channel. An oscillator of this type would generate stable frequency signals. However, the additional crystals add considerably to the cost of the modem.
Another possibility is to use a frequency synthesizer. A frequency synthesizer circuit typically has one crystal to generate a stable frequency reference and a variable oscillator to generate an output signal at selectable frequencies based on the frequency reference. The frequency of the output signal is maintained stable by a feedback loop which compares the output frequency with the reference frequency and continuously provides appropriate correction to the oscillator.
Frequently, the variable oscillator of the synthesizer circuit comprises a diode tuned resonant circuit. The diode is generally of the varactor type whose capacitance (reactance) varies in response to an applied voltage. The frequency of the variable oscillator may therefore be varied by biasing the varactor diode with the correction voltage. The change in diode capacitance causes a corresponding change in the resonant frequency of the circuit.
Three basic diode tuned resonant circuits are well known and are disclosed in U. Rohde, Digital PLL Frequency Synthesizers, Prentice-Hall, New Jersey, 1983 and reproduced in FIGS. 1a-1c. FIG. 1a is a parallel resonant circuit with a tuner diode C.sub.D and load resistor R.sub.L parallel to a series capacitor C.sub.S. FIG. 1b is a parallel resonant circuit with tuner diode C.sub.D and load resistor R.sub.L parallel to the diode C.sub.D. And, FIG. 1c is a parallel resonant circuit with two tuner diodes C.sub.D.
Each of the circuits in FIGS. 1a-1c allow the resonant frequency of the circuit to be altered. However, these circuits have the disadvantage that the range over which this alteration is useful in an oscillator circuit is relatively narrow because the Q-factor of these circuits, a measure of circuit performance, is relatively non-constant over their adjustable frequency range.